The regulations concerning satellite radio communications make it possible to deploy non-geostationary systems in frequency bands previously reserved for the systems based on geostationary satellites.
However, the coexistence of two satellite communication systems using, totally or partially, the same frequency bands, poses a problem of management of the interference generated by one system to another. In this case, the geostationary system is likely to be disturbed, even interfered with, by the transmissions between satellites and ground stations belonging to the non-geostationary system.
To guarantee a given quality of service for a geostationary communication system, the International Telecommunications Union (ITU) has set a maximum authorised interference level for the geostationary telecommunication systems as a whole and originating from systems coexisting on the same frequency bands and using non-geostationary satellites.
To ensure that a non-geostationary system does not generate an interference level above an authorised threshold, one solution consists in setting, for the set of links between the ground terminals and the non-geostationary satellites, a separation angle threshold to be observed with respect to the geostationary arc. The geostationary arc designates the view of the geostationary orbit from the earth.
A terminal of the non-geostationary system is authorised to set up a communication link (uplink, down link or two-way) with a non-geostationary satellite if, and only if, its separation angle is greater than or equal to the separation angle threshold set and independently of the other links set up by the other terminals. Thus, each terminal sees the number of non-geostationary satellites with which it can potentially communicate being reduced. In particular, the non-geostationary systems can use spectral resources in band Ka and the terminals of these systems can operate with very low elevation angles. Such conditions bring about the requirement to use very high transmission powers. Such power levels then necessitate the use of a separation angle of high value so as not to interfere with the geostationary systems co-existing on the same band Ka. Typically, a separation angle threshold value of the order of 7 degrees is used to satisfy all these constraints. Unfortunately, the use of a separation angle with a value that is fixed to a constant and high value causes the geographic coverage provided by the non-geostationary system to be reduced.
A simple solution making it possible to ensure a level of service and a geographic coverage that are sufficient for the non-geostationary system while observing the recommended separation angle threshold consists in increasing the number of non-geostationary satellites in the constellation of the system. Thus, an average number of satellites available for each terminal is assured while the terminal-satellite links which are not compatible with the separation angle threshold are prohibited.
However, the increase in the number of non-geostationary satellites presents drawbacks of cost overheads for the design of the overall system and also of sub-optimality because the set of communication resources available is not fully used because of the impediment linked to the coexistence with the geostationary systems.
One static method described in the patent application US 2003/0073404 A1 proposes avoiding the alignments of the earth/space radio frequency links between a new non-geostationary system (system B) and other existing geostationary systems (systems A) sharing the same frequency band.
This method is based on taking into account predictive constraints such as the orbits of the satellites and the radio frequency characteristics of the stations and of the satellites of the systems A and B.
This static method is valid for systems operating in the low frequency bands such as band Ku. In effect, for this type of system, the geometrical aspect alone makes it possible to statically and deterministically schedule the earth/space radio frequency links to be used by the system B without causing interference on the other existing systems A and while ensuring the quality of service expected for the system B.
This strategy is justified when the atmospheric attenuations are negligible (which is the case with the systems using low frequency bands).
This static method is however no longer appropriate for a new system B operating in high frequency bands such as band Ka. In effect, the use of low elevations combined with high atmospheric attenuations, like those induced by rain, imposes an excessive over-dimensioning in terms of power required onboard and at the ground level of the infrastructure of the system B in order to ensure the expected quality of service.
A variant of the above method consists in forcing the earth/space radio frequency links of the new system B to be able to be implemented only when the satellites of the system B are seen by the stations of the system B with a high elevation angle. This makes it possible to reduce the dynamic range of the atmospheric attenuations even in high frequency bands like the band Ka.
Nevertheless, this strategy imposes an over-dimensioning in terms of the number of satellites to be deployed by the system B in order to ensure the rate of visibility required thereby. This over-dimensioning has a strong impact on the cost and the deployment of the ground and onboard infrastructures of the system B.